Multiple-input/multiple-output (MIMO) systems offer the possibility of communication at high bit rates for multipath channels thanks to spatial multiplexing and space-time coding techniques used on transmission, without increasing the transmission band. Moreover, space-time codes (CST) also enable the range of the links to be increased and the links to be made reliable with no return loop between the receiver and the transmitter. Among space-time codes, orthogonal space-time codes (CSTO) are of very particular interest. They are designed to generate a maximum spatial diversity for a given number of transmitter and receiver antennas, and enable very simple optimum decoding (in the maximum likelihood sense). The simplest and most popular orthogonal space-time code was discovered by Alamouti [1] and has been adopted in numerous standards such as the UMTS (Universal Mobile Telecommunications Systems), DSM, EDGE, the IEEE standard 802.11 and the IEEE standard 802.16 [7]. It uses two transmitter antennas and is compatible with one or more receiver antennas.
Moreover, as spectrum is a rare and costly resource, increasing network capacity without increasing the band constitutes an undoubted challenge for cellular networks. It thus motivates the development of interference rejection techniques enabling a plurality of users to share the same spectral resources without impacting on the transmission quality for each user.
Multi-Antenna Techniques
In this context, a number of interference rejection techniques [22], [23] enabling (P+1) users to share the same channel at a given time have been developed in the last decade for users equipped with M antennas and using orthogonal space-time coding for transmission. It has been shown that in such an environment the symbols of each user may be demodulated with a diversity of order M if the receiver is equipped with N=MP+1 antennas. However, the number of receiver antennas may be reduced if the structure of the orthogonal space-time code is exploited. In this case, to obtain a diversity gain of order M and to reject P “spatio-temporal” users each having M transmitter antennas, the minimum number of receiver antennas required becomes N=P+1. Such an interference rejection structure has been proposed in [22], [23] for a receiver equipped with N=2 antennas and for P+1=2 co-channel users, each equipped with M=2 transmitter antennas and using Alamouti's orthogonal space-time code [1]. Generalization of this technique to P+1>2 users of Alamouti type with N>P receiver antennas has been proposed in the prior art. Finally, it is also known from the prior art to use an interference rejection technique enabling a receiver with N>P antennas to separate P+1 transmitted signals, each equipped with M>2 transmitter antennas and using a quasi-orthogonal space-time code whilst assuring for each user its reception with a diversity of order M(N−P).
Accordingly, receivers robust in the face of interference currently available and compatible with transmission using an orthogonal space-time code require a plurality of receiver antennas, whatever the constellation used. Moreover, the receivers available use only part of the information contained in the second order statistics of the observations. For this reason, they become sub-optimal if the part of the statistics of order two of the observations not used contains information. This is the case in particular in the presence of intranetwork interference (i.e. interference generated by the network itself) when the constellations used by the users are non-circular, such as the ASK (Amplitude Shift Keying), BPSK (Binary Phase Shift Keying) or rectangular QAM (Quadrature Amplitude Modulation) constellations. This is also the case, for all types of constellation, in the presence of interference external to the network, either non-circular or with a very narrow band.
Mono-Antenna Techniques
On the other hand, currently available interference robust mono-antenna receivers relate to mono-carrier single-input/single-output (SISO) connections. Of these techniques, those that exploit the non-circularity (or impropriety) of rectilinear constellations (with real values), such as ASK (Amplitude Shift Keying) or BPSK (Binary Phase Shift Keying) modulation, or constellations corresponding to complex filtering of rectilinear constellations such as MSK (Minimum Shift Keying) or GMSK (Gaussian MSK) modulation or OQAM (Offset Quadrature Modulation) have received very particular attention from specialists in this technical field. These techniques implant an optimum “Widely Linear” observation filter and enable separation of two users from the same antenna by using phase discrimination between the users, whence the SAIC (Single Antenna Interference Cancellation) concept. The potential of this concept combined with its low complexity are the reasons why 3G Americas presented the SAIC technology as a very strong improvement to GSM (Global System Mobile) receivers of portable type, simultaneously enabling a substantial increase in the system capacity of the GSM network. This technology was standardized in 2005 for GSM and has been operational in numerous mobile telephones since 2006. A new standardization of this concept, known as MUROS (Multi-User Reusing One Slot), is currently under investigation with the aim of enabling a plurality of GSM users to re-use the same TDMA (Time Division Multiple Access) time slot. Extension of the SAIC concept to multi-antenna reception is known as MAIC (Multiple Antenna Interference Cancellation) and is of great benefit for GPRS (General Packet Radio Service) networks in particular.
To the degree that the installation of a plurality of antennas on a mobile phone remains a technological challenge for 4th generation mobile telephones, because of problems of overall size and cost, the SAIC technology remains of great interest for 4th generation mobiles using a Wimax or LTE type OFDM (Orthogonal Frequency Division Multiplex) waveform. For this reason, extension of this technology to OFDM waveforms using ASK modulation and a single transmitter antenna, with a mono-antenna receiver, has featured in the prior art despite the fact that ASK modulation is less efficient, in energy terms, than QAM modulation with a similar number of states, supplementary degrees of freedom are available and may be exploited to reject interference on reception. Moreover, it is also known in the case of DS-CDMA networks and MIMO systems using the V-BLAST technology that transmission with real symbols and a “Widely Linear” receiver can generate a higher spectral efficiency than using a complex constellation and a linear receiver. Consequently, the use of an ASK constellation coupled to a “Widely Linear” receiver instead of a complex constellation with a linear receiver does not seem to be a limitation and may even bring advantages in terms of bit error rate and spectral efficiency.
In the context of MIMO systems, “Widely Linear” receivers have been used recently, implicitly or explicitly, to improve the reception of a user employing the spatial multiplexing V-BLAST technology. However, despite this work, extending SAIC/MAIC technologies to transmission using orthogonal space-time coding, such as Alamouti orthogonal space-time coding, has not been proposed.